Anatomy of the Upper Limb

The upper limb is a marvel of functional design — capable of the powerful grip needed to carry weight and the extraordinary fine motor precision required for microsurgery or playing a musical instrument. Its anatomy is essential knowledge for orthopedic surgeons, neurosurgeons, physiotherapists, and emergency practitioners. This guide is for educational purposes only.

## The Shoulder Complex

The shoulder complex consists of four joints working in concert: the glenohumeral joint (the primary shoulder joint), the acromioclavicular joint, the sternoclavicular joint (the only bony articulation between the upper limb and the axial skeleton), and the scapulothoracic articulation (a physiological joint). The glenohumeral joint is the most mobile joint in the body — a ball-and-socket joint where the large humeral head articulates with the shallow glenoid cavity of the scapula. This extreme mobility comes at the cost of stability, making the shoulder the most commonly dislocated joint.

Stability is provided by static restraints (glenoid labrum, glenohumeral ligaments) and dynamic restraints — primarily the rotator cuff: supraspinatus (initiates and assists abduction), infraspinatus (lateral rotation), teres minor (lateral rotation), and subscapularis (medial rotation). The supraspinatus tendon is the most commonly torn rotator cuff tendon, typically at its insertion on the greater tubercle of the humerus (the "critical zone" of relative avascularity). The long head of biceps tendon passes through the bicipital groove and into the glenohumeral joint, making it vulnerable to tendinopathy and rupture.

## Arm Compartments

The arm (between shoulder and elbow) is divided into anterior (flexor) and posterior (extensor) compartments by the medial and lateral intermuscular septa. The anterior compartment contains the biceps brachii (elbow flexion and forearm supination; innervated by the musculocutaneous nerve), brachialis (elbow flexion; the "workhorse" flexor), and coracobrachialis. The posterior compartment contains the triceps brachii (elbow extension; innervated by the radial nerve) and anconeus. The radial nerve winds around the posterior aspect of the humerus in the spiral groove — vulnerable to injury in mid-shaft humeral fractures, causing wrist drop (inability to extend the wrist and fingers).

## Forearm Muscles

The forearm contains eight muscles in the anterior (flexor/pronator) compartment, arranged in superficial, intermediate, and deep layers, and twelve muscles in the posterior (extensor/supinator) compartment. All anterior compartment muscles are innervated by the median nerve (except flexor carpi ulnaris and the medial half of flexor digitorum profundus, innervated by the ulnar nerve). The posterior compartment is innervated by the radial nerve and its deep branch (posterior interosseous nerve).

Pronation is produced by the pronator teres and pronator quadratus; supination by the supinator and biceps brachii. The interosseous membrane connecting the radius and ulna transmits forces between the two bones and provides attachment for deep muscles.

## Hand Intrinsics

The intrinsic muscles of the hand — those with both origin and insertion within the hand — include the thenar muscles (abductor pollicis brevis, flexor pollicis brevis, opponens pollicis — all innervated by the recurrent branch of the median nerve, with the exception of the deep head of flexor pollicis brevis which has dual innervation), the hypothenar muscles (innervated by the ulnar nerve), the lumbricals (1st and 2nd by the median nerve; 3rd and 4th by the ulnar nerve), and the interossei (all ulnar nerve). The lumbricals and interossei flex the metacarpophalangeal joints while extending the interphalangeal joints — the "intrinsic plus" position. Loss of ulnar nerve intrinsics produces a "claw hand" deformity (hyperextension of MCPs and flexion of IPs of the ring and little fingers).

## The Brachial Plexus

The brachial plexus provides motor and sensory supply to the entire upper limb. Formed by the anterior rami of C5–T1, it passes between the anterior and middle scalene muscles, posterior to the clavicle, and into the axilla. The plexus is organized as roots (C5–T1), trunks (upper C5–6, middle C7, lower C8–T1), divisions (anterior and posterior), cords (lateral, medial, posterior), and terminal branches.

The five terminal branches are: the musculocutaneous nerve (C5–7; anterior compartment of arm), the median nerve (C5–T1; most anterior forearm muscles, thenar muscles, lateral two lumbricals, sensory to lateral palm and radial 3½ digits), the ulnar nerve (C8–T1; flexor carpi ulnaris, medial FDP, all hypothenar and interossei muscles, medial 1½ digits), the radial nerve (C5–T1; all posterior compartment muscles, sensory to posterior limb and anatomical snuffbox), and the axillary nerve (C5–6; deltoid, teres minor, shoulder joint sensory).

Upper brachial plexus injury (Erb's palsy, C5–C6) typically follows forceful lateral neck flexion (difficult delivery, motorcycle fall) and results in the "waiter's tip" position — shoulder adducted and medially rotated, elbow extended and forearm pronated. Lower brachial plexus injury (Klumpke's palsy, C8–T1) follows traction on the abducted arm and causes intrinsic hand muscle paralysis and claw deformity, with Horner's syndrome if T1 preganglionic sympathetic fibers are involved.

## Arterial Supply

The axillary artery (continuation of the subclavian) passes through the axilla, giving branches at its three parts relative to the pectoralis minor. It continues as the brachial artery, which gives off the profunda brachii (deep brachial artery, accompanying the radial nerve in the spiral groove) before dividing at the cubital fossa into the radial and ulnar arteries. The radial artery is palpable at the wrist between the flexor carpi radialis tendon and the radial styloid — the standard pulse point. The deep palmar arch is formed predominantly by the radial artery; the superficial palmar arch predominantly by the ulnar artery.

## Common Injuries

Carpal tunnel syndrome — the most common peripheral nerve entrapment neuropathy — results from compression of the median nerve within the carpal tunnel (bounded by the carpal bones dorsally and the flexor retinaculum volarly), causing pain, paresthesia, and thenar wasting. Cubital tunnel syndrome involves ulnar nerve compression at the elbow. De Quervain's tenosynovitis affects the first dorsal compartment (abductor pollicis longus and extensor pollicis brevis tendons) at the radial styloid. Colles' fracture — the most common fracture in older adults — is a distal radial fracture with dorsal displacement of the distal fragment, typically from a fall on an outstretched hand.